Descriptions

The fact that measuring a quantum system reduces it to apparently classical behavior, eliminating the interference patterns that are a hallmark of quantumness, cries out for an explanation. That explanation has been provided by the recognition of decoherence,whereby the interference is destroyed by the very interaction that acquires information.We begin by showing how this scenario plays out in a simple analytical model—designed to be pedagogical—of measurement in a double-slit experiment. This model illustrates the continuous trade-oﬀ between information and interference in a concrete mathematical framework that makes explicit the role of measurement-induced decoherence in the process, thereby providing a natural stepping stone to a discussion of environmentally-induced decoherence and the real target of this work: the origin and accessibility of classical reality.Quantum Darwinism, building on the decoherence program, provides an explanation for the emergence of classicality within what we have come to recognize as a fundamentally quantum universe; that is, how some physical observables take on robust, objective,veriﬁable values despite the intrinsically fragile and subjective nature of observables in quantum theory. It does so by acknowledging the role of the environment as a witness,continuously monitoring certain “pointer observables” of the system, and as a communication channel, amplifying information about those observables and distributing many copies of it throughout the world. Past work in this area focused on the ability of the environment to perform this ampliﬁcation and the amount of information potentially avail-able in fragments of the environment, but little focus has been placed on the observer's ability to actually collect that information. Here we show that redundant information is available to observers even when locality prohibits them from accessing quantum correlations within their fragment; a “bit-by-bit” measurement will suﬃce with only a slight increase in the required fragment size. Moreover, we show that, except in the case of a very low entropy environment, the decoherence process that produces these objective,classical states gives rise also to a convenient classical measurement toward which local observers can evolve. Together, these results demonstrate that even local observers can,and indeed almost certainly will if given time to adapt, share in the objective, classical reality that emerges when living in a large quantum universe, thereby providing another stepping stone in the bridge that quantum Darwinism is building between our quantum universe and classical experience.